A comprehensive understanding of rock salt rheology is required for constraining laboratory derived constitutive laws that predict the long-term behavior of salt engineering structures. To reduce risks in the operation of radioactive waste repositories and solution mined salt caverns for gas- and energy storage, the processes defining salt flow in nature need to be assessed. Based on case studies (Netherlands, Israel and Romania) from various salt tectonic settings the influence of grain-size, impurity content, mechanical stratigraphy and tectono-stratigraphic inhomogeneities to salt rheology in nature is investigated. Analysis of relatively undeformed, diapiric, or detachment salt integrates seismic, microstructural and numerical analysis with digital outcrop models and further field observations. This study builds on optical microscopy of gamma-irradiated thin sections (OM-GTS ), grain- and subgrainsize piezometry, and seismic analysis and presents an integrated approach of OM-GTS to compliment further methods (electron backscatter diffraction (EBSD), x-ray diffraction analysis (XRD), scattered electron microscopy with energy dispersive x-ray analysis (SEM-EDX), magnetic properties analysis, drone-based digital outcrop models, field work, finite-element MILAMIN, MUTILS and FOLDER codes, discrete-element (DEM) ESyS particle codes).A seismic study of the Z2 Zechstein salt from the Friesland platform in the Northern Netherlands, which was not affected by later major tectonic movement, indicates that Zechstein salt deposition was syn-tectonic. Anhydrite-carbonate Z3 stringer rupturing and local deposition of Z3 salt filled depressions above active basement faults is interpreted to have major effects on the evolution of the internal structure and formation of salt pillows and diapirs. Drill-core rock salt Z2 Kristalllagen (Stassfurt Formation) samples from the same area are compared to the diapiric equivalent Kristallbrocken from Winschoten, Zuidwending and Pieterburen in the Northern Netherlands. Microstructural analysis of halite with OM-GTS and EBSD shows large disrupted Kristallbrocken megacrystals inside a fine-grained halite matrix. For fine grained salt; (1) high strains, based on ruptured and folded thin anhydrite layers, and (2) solution-precipitation creep are inferred, as seen by a lack of crystallographic preferred orientation, a lack of subgrains, and by the presence of gamma decorated directional growth rims. In comparison, megacrystals have abundant dislocation structures indicative for dislocation creep so that, based on constitutive models, a large grainsize-dependent difference in creep rates is interpreted. The integration of microstructural analysis results for co-authorship of three publications provides valuable insights for overall interpretations of these further case studies: Magnetic fabric, petrography and microstructures of diapiric salt from the Sedom salt wall in Israel is analyzed for anisotropy of magnetic susceptibility (AMS) together with OM-GTS. Microstructures indicate dominant dislocation creep deformation with minor solution-precipitation creep in halite. AMS is found to be evoked by preferred alignment of anhydrite needles that can possibly be used as strain markers. Sophisticated outcrop studies of the Romanian Mânzălești Diapir with drone-based digital outcrop models revealed low angle shear and sub-horizontal flow structures that are interpreted as salt decapitation from an overriding nappe. OM-GTS is supporting this interpretation by revealing microstructures indicative for tectonic reworking based on power-law grainsize distribution of particles in rock salt together with high differential stresses and dynamic recrystallization of halite. Microstructures that were analyzed with OM-GIT from the Ocnele Mari salt mine (Romania) suggest Newtonian viscous deformation during multilayer folding in a shear zone. In this regard it is shown that layers with small amounts of impurities can significantly change the viscosity of rock salt so that it is mechanically stratified and flows anisotropically. Based on multilayer fold shape analysis and comparison to numerical modelling, with finite-element MILAMIN, MUTILS and FOLDER codes, the case specific flow anisotropy is estimated between δ_max = 3.0 to 5.5 and the impurity related viscosity ratio in the Ocnele Mari salt is estimated to be within the range of 10 to 20. Contrary to other studies it is found that darker layers containing impurities are more competent. Furthermore, it is observed that thin layers organize themselves into effectively single layer packages (also at micrometer scale) – even when they are discontinuous. Yet unpublished preliminary results of ESyS particle discrete-element viscous-dashpot material (DEM) calibrations for brittle-viscous flow are included in this thesis, which suggest that the material can deform dilative or compressive and might not be suitable to model deformation of materials with large viscosity contrast, although, being a suitable analogue for salt deformation as previously shown.Piezometry for diapiric Kristallbrocken salt indicates deviatoric stresses between 0.5 and 2.0 MPa and high stress of up to 4.0 MPa for Mânzălești salt in line with the interpretation of an overriding nappe. For the Ocnele Mari salt the piezometry indicates about 1.0 MPa deviatoric stresses which is interpreted to represent stresses during folding and after shearing. Possibilities of further research based on these results are included.